I use mail alias a lot. I have a unique address for almost all my services. I like this feature a lot, but it's annoying that every time I create a new alias, with a new contact asociate with it, I have to copy the address proton pass or simplelogin create and put it into the address box.
I think it's a way better to INTEGRATE contacts from ALIAS into the contacts section of the mail, just to select that contact (the one attached to the alias) just as normal contact. In fact, the better way to make alias really useful it's to integrate making alias and sync alias into the contacts app in a bidirectional way: from contacts to alias AND from alias to contact